The bearing apparatus for a vehicle wheel includes various types in accordance with applications for driven as well as driving wheels. For example, a conventional bearing apparatus for a vehicle driving wheel is shown in FIG. 5. The apparatus includes an inner member 50, a wheel hub 51, an inner ring 52, double row rolling elements 53 and 54, an outer member 55, and a constant velocity universal joint 56. The constant velocity universal joint transmits an engine output to the wheel hub 51. In such a bearing apparatus for a driving wheel, the wheel hub 51 that supports a wheel (not shown) and a brake rotor 57 is made of medium carbon steel, such as S53C, for its ease of forging, cutting, heat treatment and economy in production. The wheel hub 51 and the wheel mounting flange 58 are ribbed and thinned to make them small and light to improve fuel consumption as well as driving stability. However, since the mechanical strength of such a miniaturized wheel hub 51 itself nears the fatigue limit of the medium carbon steel forming the wheel hub 51, it is difficult to further proceed with the miniaturization and lightening of the wheel hub 51.
In the wheel hub 51, where the wheel mounting flange 58 is thinned for weight purposes, special countermeasures are required to deal with the concentration of rotary bending stress at a corner 61 extending from a brake rotor mounting surface 59 toward a cylindrical pilot portion 60. Although it is possible to reduce the generated stress by enlarging the dimension of the corner 61, by increasing the radius of curvature, it is also limited by interference due to the to be mounted brake rotor 57.
The applicant of the present application has proposed a vehicle wheel bearing apparatus which increases the strength of the wheel hub 51 as well as lightening it without changing the configuration and dimension of the wheel mounting flange 58. In this bearing apparatus, as shown in FIG. 4, the corner 61 of the flange 58 of the wheel hub 51 is formed with a surface hardened layer 62 by high frequency induction hardening. This strengthens the corner 61 of the flange 58 where the rotary bending strength is weakened most and thus increases the durability of the wheel hub 51.
Portions other than the corner 61, such as a seal land portion where a seal lip is fitted into the outboard side end of the outer member 55 (not shown in FIG. 4) as well as portions “a”˜“d” are also formed with a surface hardened layer 63. In addition, a serrated portion 64 is formed with a surface hardened layer 65. Thus the rotary bending strength, wear resistance and rolling fatigue life required for these portions “a”˜“d” can be improved. See: Japanese Laid-open Patent Publication No. 87008/2002 (pages 4 and 5 and FIG. 2).
Although it is possible, according to the bearing apparatus for a prior art vehicle wheel, to increase the strength of the hub wheel 51 as well as to lighten it without changing the configuration and dimension of the wheel mounting flange 58, by forming the surface hardened layer 62 at the corner 61 of the flange 58, new problems occur. One problem is that the wheel mounting flange 58 is deformed by heat treatment in the high frequency induction hardening step. Thus, a large run-out surface is created in the brake rotor mounting surface 59. The problem of surface run-out is also caused by thinning the wheel mounting flange 58. The surface run-out of the brake rotor mounting surface 59 influences the run-out of the brake rotor 57. This causes brake judder which impairs the driving stability and the driving feeling. In this case, although it is conceivable to further cut the brake rotor mounting surface 59 by lathe turning after heat treatment of the wheel hub 51 to modify the deformed portion to improve the surface run-out, there also remains a dilemma that a slight step is caused between the surface hardened layer 62 of the corner 61 and the unhardened brake rotor mounting surface 59 due to a difference in hardness therebetween.